# BlackHole Gravitation

#### Kannan Kailas

Hi All,

Blackholes will curve the space-time in a way so that even particles moving with speed of light are trapped. It's presence is detected by the gravitational force it exerts. So if there were particles like gravitons which would defenitely travel less than or equal to 'c', those also would have been trapped. So how exactly the gravitational force is exerted? If it was with particles like gravitons, it should move faster than light. If there are no particles, gravity can be instantaneous, because there is nothing to limit it, there by violating principle of relativity.

Or if there are gravitons which travels at 'c', there can be lot of mass whose presence can never be proved and our understanding of universe is wrong as far as with matter density...

Can anyone explain me where I went wrong in these wild thoughts?

Kannan Kailas.

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#### marcus

Gold Member
Dearly Missed
Originally posted by Kannan Kailas
Hi All,
So how exactly the gravitational force is exerted? If it was with particles like gravitons...
Hello Kannan, and welcome.
I wont try to give a complete answer and hope some of the others reply to your question more thoroughly.

I share your dissatisfaction or puzzlement about gravitons. They dont work for me as a way of understanding how gravitational force is exerted.

We all have the idea of electrostatic force being exerted by the exchange of virtual photons----this picture is firmly set in our minds so that we want to understand gravity the same way, but it does not work so well. The idea of "graviton" may turn out to be a low energy approximation to disturbance of the geometry that works only when the geometry is nearly flat. That is, it may be less of a real thing and more of an artifact of the mathematics appropriate to a limited situation.

It may be best to think of the gravitational field surrounding a black hole NOT as made up of the exchange of gravitons but in a geometric way as arising from spacetime curvature.

If something falls into the black hole, changing the distribution of mass momentarily, then the field will ripple and the information about this will gradually spread out into the universe-----but maybe in the highly curved space surrounding the hole it doesnt work so well to describe the ripples as "gravitons"

Maybe it is best just to think of the field as a dynamic geometry which can undergo change and undulate (but not try to analyze this into particles).

But I still sense that there is a core part of your question that is deeper. How can these ripples in the field escape, even if we dont call them "gravitons"?
I think the answer is that when something falls into a BH, or when two BHs collide and merge into one, then the disturbance to the field arises already OUTSIDE the event horizon, in the changing shape of space around, but outside. That information can spread out.

It is only what happens INSIDE the event horizon that we can never learn about.

When finally human instruments like LIGO are good enough to detect the gravity wave caused by something falling into a BH or suchlike events, what we will be "seeing" is the momentary disturbance of the shape of space that happened JUST OUTSIDE the event horizon, and so the information could still get away even though it could not travel faster than c.

Maybe someone else will give a more precise reply.

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#### wolram

Gold Member
hope you dont mind this post, i read this a short time ago
and thought it may be of interest, seems they just canot
find them little Bs.

http://curious.astro.cornell.edu/question.php?number=264 [Broken]

You may have heard about how space is a "frothing sea" of elementary particles, with particles and antiparticles continuously popping into and out of existance. That would be another manifestation of virtual particles. Virtual particles are essentially allowed to do anything at all short of violating causality during their lifetimes. They necessarily violate conservation of energy just by existing, and they're also allowed to violate many other physical laws before they disappear. One of these is the restriction on travelling slower than the speed of light. So the virtual gravitons and photons speed away from the singularity faster than light until they're beyond the event horizon, from whence they spread out into the universe to carry on their virtual business. Various physical processes conspire to insure that no actual information is transmitted superluminally; that is, if the black hole were to instantly disappear, its gravity would still be felt outside a radius ct from the singularity.

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